Editor’s Note: Peter Bergen is CNN’s national security analyst, a vice president at New America and a professor of practice at Arizona State University. The views expressed in this commentary are his own. View more opinion on CNN.

CNN  — 

Has Vladimir Putin’s assault on Ukraine outrun the resources he’s committed to it? That’s the view of retired US Army Major General Mike Repass, who has an informed vantage point on the conflict, having worked in the Ukrainian security sector since 2016. The former commander of the US Special Operations Command in Europe, Repass provides education and advisory support to the Ukrainian military on a US government contract.

Mike Repass
Peter Bergen

In discussions Thursday and Friday, I spoke to Repass about why new leadership and the improved training of the Ukrainian military has markedly improved its performance in recent years, the kind of anti-aircraft and anti-tank weapons the Ukrainians hope that the US and its NATO allies will supply them with and what he sees happening next as the war in Ukraine grinds on. He predicts a campaign by the Russians that could turn the cities of Ukraine into rubble, creating a refugee crisis that overwhelms bordering nations, and destabilizes Central and Eastern Europe.

But Repass believes that while the Russians may be able to overcome Ukraine’s stiff defense, they will not be able to hold onto the country because Putin doesn’t have sufficient forces in theater to occupy large swaths of Ukraine indefinitely. In short, Putin has bitten off more than he can chew.

Disclosure: Repass is on the advisory council of the Global Special Operations Foundation, where I am the chairman of the board. Our conversation was edited for clarity and length.

Repass: The bottom line is the Ukrainian military forces have acquitted themselves exceptionally well thus far in the war. Russia will have a very difficult time subduing them because they are willing to fight until it becomes seemingly “futile,” or they no longer have the resources to do so.

The Ukrainians have been overmatched by Russian technology and outmanned and outgunned – by Russian tanks, artillery, precision long range strike missiles, armored personnel carriers – but the terrain favors the defenders, especially in the north and east of Ukraine, although less so in the south.

I think time and mass are on the Russian side, and they’re going to be able to either create conditions for peace suitable to Putin’s liking, or they will outright destroy the cities of Ukraine and the Ukrainian military with it, which to me still leaves a resistance scenario for the Ukrainians. So, there are multiple plausible futures.

Bergen: Why are the Ukrainians fighting better than many had expected?

Repass: I’m not surprised at how well the Ukraine army is fighti